This method searches the genome for single nucleotide polymorphisms (SNPs) in any gene that occur more frequently in people with a particular disease than in people without the disease.[17,18]Offered when there is an increased risk of having a child with a genetic condition due to maternal age, family history, ethnicity, or suggestive multiple marker screen or fetal ultrasound examination. A relatively short single-stranded nucleic-acid chain usually consisting of 2 to 20 nucleotides that is synthesized to match a region where a mutation is known to occur, and then used as a probe. The balance of benefits and harms associated with the use of a genetic test in practice, including improvement in measureable clinical outcomes and usefulness/added value in clinical management and decision-making compared with not using the test.
Preimplantation testing provides an alternative to prenatal diagnosis and termination of affected pregnancies.In these studies, the candidate genes must be known a priori and are therefore limited by understanding of the genes that contribute to a particular disease.Genetic association studies have been limited by their lack of reproducibility.While these data have the potential to inform investigations into disease etiologies and thereby advance medicine, the question of how to adequately control both false positive and false negative rates remains. Genome-wide association studies are a relatively new way for scientists to identify genes involved in human disease.
 Nucleotides: The monomeric units from which DNA or RNA polymers are constructed.